With the constant increase of mobile data services, the 3rd Generation Partnership Project (3GPP) organization has developed long-term evolution (LTE) specifications and LTE-Advanced (LTE-A) specifications. As the next generation cellular communication standard, an LTE or LTE-A system can operate in both Frequency Division Duplex (FDD) mode and Time Division Duplex (TDD) mode.
Machine-to-Machine (M2M) communication, which may also be called as Machine-Type Communications (MTC), is an emerging communication pattern. It refers to communication between computers, embedded processors, smart sensors, actuators and mobiles devices without or with only limited human intervention and it is quite advantageous in many applications such as sensing in extreme or hazard environment. Generally, many of MTC UEs are targeted for low-end applications (low average revenue per user, and low data rate) that can be handled adequately by GSM/GPRS and thus they may be implemented at low cost.
As LTE deployments evolve, it is desirable to reduce the cost of overall network maintenance by minimizing the number of Radio Access Technologies (RATs). However, there are deployed more and more MTC UEs in the field, which increases reliance on GSM/GPRS networks, and thus cost for operating these networks are increased. Hence, it will be very beneficial if low-end MTC UEs may be migrated from GSM/GPRS to LTE Networks.
It is known that in LTE releases 8 to 11, data transmission is designed for medium or high SNR (SNR>−5 dB), but SNR of MTC UE could be as low as −25.3 dB. Accordingly, it is rather challenging to support MTC UEs in current Available LTE release.
Regarding the MTC UE migration, the 3GPP has started a study item to study a possibility to support MTC UEs in a low SNR region. FIG. 1 illustrates a mapping of uplink demodulation reference signals (DMRS) as proposed in 3GPP TS 36.211. In this figure, each block represents a resource element (RE) and REs in two symbols are used to transmit uplink DMRS and other REs are used to transmit data. However, the DMRS configuration is designed for medium or high SNR. Whereas the MTC normally operates with a low SNR and thus the DMRS pattern is not suitable for the MTC at all.
In 3GPP technical document TR 36.888, there are proposed approaches to support low-cost MTC UEs based on LTE, wherein the possibility of increasing the density of reference signal is discussed. It is known that the number of retransmission/repetition for MTC UE largely relies on the accuracy of channel estimation. Increasing the density of reference signal in time domain can improve the accuracy of the channel estimation in low SNR. However, it will increase the overhead definitely if the density of reference signals is increased, which will in turn increase the coding rate. This means that usually, increasing density of uplink DMRS might not obtain performance gains as expected.
Therefore, there is a need for a new solution of uplink data transmission in a wireless communication system for improving accuracy of channel estimation in a low SNR communication such as MTC.